An ultra-luminous cluster is a young globular cluster, that has formed in the past few tens or hundreds of millions of years, and that is composed mainly of bright blue stars. Ultra-luminous clusters are several hundred times brighter than the ancient variety found in the Milky Way and are characteristic of galaxies that have recently undergone collisions or mergers, such as the Atoms for Peace Galaxy and the Antennae, and are also believed to have been common in galaxies in the early universe.